DESCRIPTION: The goal of the proposed research is to understand the vestibulo-cervical reflex, also called the vestibulo-colic reflex (VCR). The neuronal circuitry of the VCR receives vestibular information from the semicircular canals and otoliths, and coordinates neck muscle contraction in response to vestibular stimuli. When the VCR is disabled the head wobbles rapidly and uncontrollably, demonstrating that one VCR function is to maintain head stability. Head and body righting reflexes are also disrupted by interruption of vestibular signals, suggesting that another VCR function is righting the head to maintain the usual upright posture. The proposed research to be done in squirrel monkeys and cats has two major hypotheses with corresponding experimental aims: 1. The Vestibulo-Cervical Reflex returns the head to the upright position: when the head is upright and intermediate frequencies of head motion are tested, a stabilizing VCR responses is observed. However, when the head is not upright and low frequencies are tested, responses can oppose head stability if such opposition would facilitate righting of the head. Neck muscle electromyographic activity and brainstem VCR neuron activity will be recorded during low-frequency head rotations in a variety of head postures to determine when stabilizing responses occur and when righting responses are elicited by head motion. 2. Vestibulo-Cervical Reflex signals can dominate vestibulo-ocular reflexes: Reflex eye movements will be recorded at the same time as neck muscle electromyographic and VCR neuron activity to determine if ocular reflexes are altered by the transition of VCR response from a stabilizing to righting function. Activity of neurons participating in both reflexes will be recorded to see which reflex dominates.